1. Field of the Invention
This invention relates to methods for forming a large area amorphous semiconductor layer and for producing a contact type image sensor, and also to a contact type image sensor thus produced.
2. Description of Prior Art
In a data input device of a facsimile apparatus or copying machine and the like, a contact type image sensor with a large sized readout element having a width substantially equal to that of an original copy and hence requiring no optical system for reducing images has been used for the purpose of reducing the size of the apparatus and simplifying related mechanisms.
As shown in FIG. 1, the contact type image sensor typically comprises a photoelectric converting element portion L and a driving circuit portion D for driving the photoelectric converting element portion L. The photoelectric converting element portion L comprises a lower electrode 2 made of a thin film of a metal such as chromium, which is dividedly formed on a glass substrate 1, a photoconductive layer 3 made of a thin layer of hydrogenated amorphous silicon, and an upper transparent electrode 4 made of indium-tin oxide (ITO) or the like, these electrodes and layer being laminated in the aforementioned order in a sandwich manner. Since no optical reduction system is necessary with the contact type image sensor, the width of the photoelectric converting element portion L must be equal to or larger than the width of the original copy. Thus, readout elements having widths of 210 mm and 256 mm are required to cover the original copies of No. 5 size of B series and No. 5 size of A series of the Japanese Industrial Standard (JIS), respectively. In the readout element, 1728 and 2048 sandwiched type photoelectric conversion elements constituting 1728 and 2048 bits are arranged in an array, respectively, and in order to read data correctly, these elements must be of precisely equal properties.
Ordinarily, the production of such an elongated readout element comprises the processes of forming a lower electrode in which a thin film of a metal such as chromium (Cr), nickel (Ni), platinum (Pt) and the like is formed by the vapor deposition method or the like on an inflexible insulating substrate 1 made of ceramic, glass, plastic or the like; forming the thin film into a desired pattern by the photolithoetching method; forming a photoconducting layer in which a photoconductive layer such as a hydrogenated amorphous silicon layer (s-Si:H) and the like is deposited by the glow discharge method; and forming a transparent electrode using a transparent material such as indium-tin oxide (ITO) by, for example, the sputtering method. To provide an element having uniform properties for each bit, the electrode films and the amorphous layer formed by the above described processes must be uniform throughout the entire surface of the substrate. Since an entire substrate becomes defective by the presence of only one defective bit, the formation of the films and the layer must be executed with great care to prevent the reduction of the yield.
In a contact type image sensor where uniform films and layer must be assured, the apparatus for fabricating the sensor becomes larger as the substrate becomes larger. Particularly in the process for depositing the amorphous semiconductor layer as a photoconductive layer by using plasma CVD apparatus, gas plasma flows unevenly at the end portions of parallel arranged planar electrodes for glow discharge in the plasma CVD apparatus. Thus, these end portions cannot be used to form a uniform layer. Therefore, still larger apparatus is required. To read documents of JIS B series No. 4 size for example, a readout element must be wider than 280 mm. However, apparatus now available cannot fabricate an image sensor whose readout portion is wider than 280 mm.